You need a bunch of solutions to make one UI that works for both landscape and portrait.
It is doable however, often a layout within an aspect fitter, for instance.
Here are some notes on UI Anchoring, Scaling, CanvasScaler, etc:
Usually you need to choose a suitable ScaleMode and MatchMode in the Canvas Scaler and stick with it 100%. Generally if you change those settings you will often need to redo your UI entirely.
I also use this CanvasScalerOrientationDriver utility to make sharing UI for Landscape / Portrait easier. Read what it does carefully.